Combined cycle power plants have come into widespread use because they incorporate heat exchangers that can recover heat from the hot gas exhaust stream of a combustion engine. Conventionally the recovered heat is used to generate the working fluid of a steam turbine. This results in more efficient power generation than is achievable with only a combustion turbine or only a steam turbine. See, for example, U.S. Pat. No. 5,375,410 which is assigned to the assignee of the present invention and incorporated herein by reference.
Generally, combined cycle power generation systems include a first power source based on a combustion process and a second power source which operates on a Rankine cycle, i.e., a steam cycle. Conventionally the first power source is a gas turbine, wherein heat from hot exhaust gases of the combustion process are transferred to the working fluid in the Rankine cycle through a Heat Recovery Steam Generator (HRSG). Such systems can operate at overall plant efficiencies on the order of 55 percent or higher.
Combined cycle power generation systems are most efficient during steady state operations. However, at times of peak power demand there are often needs to rapidly increase power output. One method for doing so, commonly referred to as power augmentation, involves diversion of steam from the steam cycle, e.g., removal of superheated steam prior to output from a steam turbine, and feeding the steam directly into the combustion chamber of the gas turbine. When a combined cycle power plant operates in such a power augmentation mode, the steam removed from the Rankin cycle is at a pressure somewhat higher than the pressure of the compressed air at the input to the combustion chamber. Once the steam enters the chamber its temperature is substantially elevated as it mixes with the combustion gases. This results in substantial expansion such that power output from the steam via the gas turbine section is much greater than would be provided with the steam turbine.
However, diversion of the steam for power augmentation removes energy from the Rankine cycle during the same period in which efforts are undertaken to increase plant power output. Thus efforts to move the system into a higher level of steady state power output are impeded because the lost steam must be replaced by heating relatively cool make-up water. The amount of make-up water required can be 20 percent or more of the feedwater volume present in the Rankine cycle.
Energy losses associated with steam diversion and injection of make-up water are compounded when, as is often the situation, the make-up water is of insufficient purity. In a once-through design, to remove impurities steam bottles are commonly incorporated in one or more stages of the HRSG to effect moisture separation. Although the flow is normally controlled to create dry steam at the exit of the HRSG evaporator, by increasing the volume flow rate of the feedwater flowing from the boiler and through the evaporator, the steam remains moist as it exits the HRSG evaporator tubing. Steam bottles placed in-line with the tubing facilitate removal of the moisture and, along with the moisture, a substantial portion of the impurity component is also removed. This may effect removal of about 90 percent of the impurities but with further loss of energy present in the separated moisture.
In other HRSG designs, moisture separation is continuous in the steam drum, which always contains liquid.
Like reference numbers are used to denote like features throughout the figures.